Is It Possible...

to stack an 8 inch inch tier (4 inches tall) on top of a 6 inch tier of the same height?

I have this ambitious idea for a cake for a cake competition at the end of April. I'll be using dummies but can't do anything with them that I can't do with real cake. I'm assuming that with the right support you can but I'm having a hard time finding examples. Any help would be greatly appreciated!

Yes, you can do it. There are amazing decorators who do "upside down" tier cakes as you describe. I frankly have no idea how they set up their multi-tier structures. But for your purpose, I can suggest that you'll need a sturdy cardboard cake circle under your 8" tier to support the overhang. Then use whatever dowels/straws you'd normally use in your 6" tier (though I've never used dummies do I don't know if you need internal support) . You'll certainly need to work on getting the balance perfect, and I'm sure there will be other issues that will arise. But it sounds like fun. Good luck!